This application is related to Japanese application No. 2000-327155 filed on Oct. 26, 2000, whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a magneto-optical recording medium to be embodied as a magneto-optical disk, a magneto-optical tape, a magneto-optical card or the like for use with a magneto-optical recording/reproducing apparatus, and method of reproducing the same.
2. Description of the Related Art
In recent years, magneto-optical recording media have come into limelight as external recording devices for computers. Such a magneto-optical recording medium, which is adapted to form submicron recording bits thereon by application of an external magnetic field and a laser beam, has a greater recording capacity than conventional types of external recording media such as floppy disks and hard disks.
A currently available 3.5-inch magneto-optical recording medium, for example, has 1.1-xcexcm pitch tracks provided on an area thereof defined between 24-mm radius and 40-mm radius concentric circles, and is adapted to circumferentially write marks of a minimum size of 64 xcexcm to provide a recording capacity of about 640 MB on each side thereof. The magneto-optical recording medium is a rewritable medium having a very high recording density.
However, the recording capacity should further be increased to record a tremendous amount of data and motion pictures for the upcoming multimedia age. For the increase in the recording capacity, a greater amount of recording marks should be formed on the medium. Therefore, marks having a smaller length should be arranged at smaller intervals than the currently employed marks. For higher density recording with this arrangement, a laser beam to be applied to the medium should have a wavelength smaller than 780 nm or 680 nm. In consideration of the practicality, the reduction in the length of the marks is more effective than the reduction in the wavelength of the laser beam.
Various methods have been proposed for data reproduction from marks having a smaller size than the diameter of the laser beam.
Japanese Unexamined Patent Publication No. HEI 1(1989)-143041, for example, proposed a method called xe2x80x9cFAD (Front Aperture Detection) methodxe2x80x9d (first prior art), which is adapted to read a recording mark in a low temperature region within a laser spot while utilizing a high temperature region as a mask region.
Japanese Unexamined Patent Publications No. HEI 3(1991) -93056 and No. HEI 3(1991)-93058 proposed methods called xe2x80x9cRAD (Rear Aperture Detection) methodxe2x80x9d (second prior art), which is adapted to read a recording mark in a high temperature region within a laser spot while utilizing a low temperature region as a mask region.
Japanese Unexamined Patent Publication No. HEI 4(1992)-271039 proposed a method called xe2x80x9cRAD double mask methodxe2x80x9d (third prior art), which is adapted to read a recording mark in an intermediate region between a low temperature region and a high temperature region within a laser spot while utilizing the low temperature region and the high temperature region as mask regions.
Japanese Unexamined Patent Publication No. HEI 5(1993)-12731 proposed a method called xe2x80x9cCAD (Center Aperture Detection) methodxe2x80x9d (fourth prior art).
The prior art methods described above can read a recording mark in a region having a smaller size than the diameter of a spot of a reproduction laser beam, and provide a resolution substantially equivalent to that provided by reproduction with the use of a light spot smaller in diameter than the spot of the reproduction laser beam.
However, the aforesaid prior art methods have the following drawbacks.
The first prior art method, which is adapted for reproduction in the low temperature region, allows for size reduction of the entire system without the need for provision of an initialization magnet, but is not effective for prevention of crosstalk because recording marks in neighboring tracks may be detected to affect the reproduction.
The second prior art method, which is adapted for reproduction in the high temperature region, is effective for prevention of crosstalk, but does not allow for size reduction of the system with the need for provision of an initialization magnet.
The third prior art method is also effective for prevention of crosstalk, and allows for enhancement of reproduction output. However, it is impossible to reduce the size of the system with the need for provision of an initialization magnet as in the second prior art method.
The fourth prior art method requires no initialization magnet, but fails to provide a high reproduction output because there is a larger transition area in which the orientation of magnetization of a reproduction layer is shifted from an in-plane direction to a perpendicular direction.
Since the prior art methods have the drawbacks described above, the inventors of the present invention have proposed, in Japanese Unexamined Patent Publication No. HEI 7(1995)-244877, a magneto-optical recording medium (fifth prior art) which is capable of providing a magnetic super resolution (MSR) and a high reproduction output without the need for provision of an initialization magnet. An explanation will hereinafter be given to the magneto-optical recording medium according to the fifth prior art.
As shown in FIG. 10, the magneto-optical recording medium comprises a reproduction layer 4, an intermediate layer 5 and a recording layer 6 stacked in this order on a substrate (now shown). The reproduction layer 4 is composed of a rare earth-transition metal amorphous alloy such as GdFeCo, and has a direction of easy magnetization extending perpendicularly thereto. The intermediate layer 5 is composed of a rare earth-transition metal amorphous alloy such as GdFeCo, and has a direction of easy magnetization which extends in an in-plane direction at room temperature but is shifted from the in-plane direction to a perpendicular direction when the layer is heated up to a predetermined temperature by application of a reproduction light beam. The recording layer 6 is composed of a rare earth-transition metal amorphous alloy such as TbFeCo, and has a direction of easy magnetization extending perpendicularly thereto. The reproduction layer 4, the intermediate layer 5 and the recording layer 6 have Curie temperatures Tc1, Tc2 and Tc3, respectively, which satisfy relationships of Tc2 less than Tc1 and Tc2 less than Tc3. Further, the reproduction layer 4 and the recording layer 6 have coercive forces Hc1 and Hc3, respectively, which satisfy a relationship of Hc3 greater than Hc1 at room temperature.
The reproduction layer 4 serves as a mask for reading a signal or for providing a magnetic super resolution. The intermediate layer 5 has an in-plane magnetization property at room temperature and, when the layer is heated, is exchange-coupled to the recording layer 6, whereby the magnetization direction thereof is copied to the reproduction layer 4. The recording layer 6 is adapted for thermal magnetic recording which is achieved by heating the layer up to a temperature near its Curie temperature with application of a recording magnetic field for inversion of the direction of the magnetization.
For reproduction of data recorded in the recording layer 6, smaller size marks are accurately read by utilizing a temperature gradation generated within a laser spot on the medium.
Erasing, recording and reproducing operations to be performed on the magneto-optical recording medium will be explained with reference to FIGS. 10 to 13. It is herein assumed that an upward bias magnetic field is applied for recording data and a downward bias magnetic field is applied for reproducing and erasing the data. The explanation will be given on the assumption that the reproduction layer 4 and the recording layer 6 are rich in transition metals (TM-rich) and the intermediate layer 5 is rich in rare earth elements (RE-rich).
As shown in FIG. 10, the magneto-optical recording medium is irradiated with an erasing laser beam with a bias magnetic field (erasing magnetic field 16) being applied downward, so that the recording layer 6 is heated at a temperature higher than its Curie temperature thereby to be magnetized downward. The recording medium is cooled to room temperature when it is brought away from the laser beam. At room temperature, the intermediate layer 5 is in an in-plane magnetization layer, so that the reproduction layer 4 and the recording layer 6 are not magnetically coupled to each other.
Therefore, the reproduction layer 4 is uniformly magnetized downward by application of a magnetic field having a relatively low intensity equivalent to that of the erasing bias magnetic field.
As shown in FIG. 11, only a recording site of the recording medium is irradiated with a high intensity laser beam with a bias magnetic field (recording magnetic field 17) being applied upward, whereby only the data recorded site is magnetized upward. The recording medium is cooled to room temperature when it is brought away from the laser beam. At room temperature, the intermediate layer 5 is in the in-plane magnetization layer, so that the reproduction layer 4 and the recording layer 6 are not magnetically coupled to each other. Therefore, the reproduction layer 4 is uniformly magnetized downward by application of a magnetic field having a relatively low intensity equivalent to that of the bias magnetic field.
Next, an explanation will be given to the reproducing operation. In a low temperature region of a laser spot within a region 20 to which a reproducing magnetic field 14 is applied, the exchange-coupling force between the intermediate layer 5 and the recording layer 6 is weak, so that the magnetization of the intermediate layer 5 is oriented in the direction of the reproducing magnetic field and the magnetization of the reproduction layer 4 is oriented upward opposite to the direction of the magnetization of the intermediate layer 5 by an exchange-coupling force (front mask 13a). In a high temperature region, on the other hand, the intermediate layer 5 and the recording layer 6 are exchange-coupled to each other, and the intermediate layer 5 and the reproduction layer 4 are exchange-coupled to each other as shown in FIG. 12. Therefore, the magnetization direction of the recording layer 6 is copied to the reproduction layer 4, so that data recorded in the recording layer 6 can be read. This reproducing process is referred to as xe2x80x9csingle mask reproductionxe2x80x9d.
In the high temperature region, the recording medium is heated at higher than the Curie temperature of the intermediate layer 5, so that the reproduction layer 4 is uniformly magnetized downward in the direction of the bias magnetic field as shown in FIG. 13. Therefore, the reproduction layer 4 serves as a mask (rear mask 13b). This reproducing process is referred to as xe2x80x9cdouble mask reproductionxe2x80x9d.
When a magneto-optical output is differentially detected, a magneto-optical signal is read neither in the low temperature region nor in the high temperature region, but only in an intermediate temperature region, because the low temperature region and the high temperature region serve as the masks within the laser spot. This allows for super resolution reproduction and provides a high reproduction output without provision of an initialization magnet, so that data can accurately be reproduced from marks having a size smaller than a diffraction limit of the wavelength of the laser beam. In FIGS. 10 to 13, a reference character A denotes a moving direction of the medium, and reference characters 12 and 18 denote an aperture and a beam spot, respectively.
However, it has been found that, where the use of a land/groove substrate having a smaller track pitch is desired for higher density recording, for example, the prior art methods present the problem of crosstalk. This is attributable to the fact that, with a track pitch smaller than the diameter of the beam spot, the heat reaches neighboring tracks, whereby recording marks on the neighboring tracks are also brought into a magnetization copied state.
Where the laser beam has a wavelength of 660 nm and an objective lens has an NA of 0.55, for example, the beam spot has a diameter of about 1 xcexcm. When a land/groove substrate having a track pitch of 0.6 xcexcm is employed in this case, about 40% of light of the beam is applied to the neighboring tracks to cause crosstalk even with a magnetic super resolution.
The medium for the magnetic super resolution has a higher circumferential resolution to make it possible to read small marks. In order to reduce the width of the tracks for increasing the density of the recording medium, it is necessary to increase a radial resolution.
To solve the aforesaid problem, the present invention is directed to a magneto-optical recording medium which is highly resistant to crosstalk and permits the use of a land/groove substrate having narrower tracks for improvement in recording density.
In accordance with the present invention, there is provided a magneto-optical recording medium which comprises four magnetic layers including a mask layer, a reproduction layer, an intermediate layer and a recording layer, wherein the reproduction layer and the recording layer each have a direction of easy magnetization extending in a layer stacking direction at room temperature, the mask layer and the intermediate layer each have a direction of easy magnetization extending in an in-plane direction at room temperature, the mask layer, the reproduction layer, the intermediate layer and the recording layer have Curie temperatures Tc1, Tc2, Tc3 and Tc4, respectively, which satisfy relationships of Tc3 less than Tc2, Tc3 less than Tc4 and Tc3 less than Tc1, and the intermediate layer is a rare-earth-rich magnetic layer.
In accordance with the present invention, there is provided a reproduction method for the magneto-optical recording medium which comprises applying a laser beam to the magneto-optical recording medium while applying a reproducing magnetic field thereto to form a high temperature region, a medium temperature region and a low temperature region within a beam spot, and reproducing data from the medium temperature region with use of the low temperature region and the high temperature region as masks.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.